1. Field of the Invention
This invention relates to surface topography and more particularly to a method for forming a serrated surface topography.
2. Description of the Prior Art
Bubble domains are propagated by (a) rotating field propagation systems having T and I bars or chevron permalloy overlay patterns (b) modulated bias field propagation systems having angel fish patterns etched in the bubble material or deposited as a permalloy film on top of the bubble material and (c) current loop propagation systems.
An alternate modulated bias field propagation system approach was proposed in the patent to Bobeck et al. U.S. Pat. No. 3,540,019. This propagation system utilizes a serrated surface topography as shown in FIG. 1. The serrated surface topography produces unidirectional bubble domain translation when the bias field is modulated. The serrated surface topography consists of a periodic pattern of serrate grooves running perpendicular to the direction of the bubble domain translation. It is the purpose of this serration to produce a unidirectional wall motion coercive force perpendicular to the grooves. A unidirectional coercive force for wall motion arises because the energy of the domain wall is proportional to its height, which equals the local film thickness. The sloped surface of the serration thus exerts a force on any domain wall located under such a sloped position, tending to move the wall toward the low point of the slope. The magnitude of this force is proportional to the surface slope and is largest for a wall segment oriented parallel to the serration direction. No force is exerted on a wall segment which is oriented perpendicular to the serration direction. It can be seen that a small force suffices to push a wall segment up the gentle slope 80, while a much larger force is required to push a wall segment up the steep slope 76 as shown in FIG. 1. On varying the bubble size by at least one serration period by means of a bias field modulation, the serration acts much like a ratchet on the bubble wall and thus propagates the bubble in the gently increasing slope direction. The term "unidirectional coercive force" is used to specify the difference in force required to move a bubble in the forward and the backward direction and describes the efficiency of the serration topography. The unidirectional coercive force is largest with a backward slope of 90.degree. as shown in U.S. Pat. No. 3,540,019.
In a typical bubble material, such as Europium Yttrium Iron garnet, having a characteristic length, l, to thickness, h, ratio of 0.25, where l and h are two well known material parameters, the bubble size within a bubble lattice varies typically between 4.mu. and 8.mu. when the bias field is varied between 240e and 180e. The 4.mu. modulation in bubble size thus allows for a maximum serration period of 4.mu.. More reliable lattice propagation is obtained when the serration period is somewhat smalles than the modulation in bubble size.
When a serrate surface topography is used to propagate isolated bubble domains, the width of the serration needs to be substantially equal to the bubble domain diameter. If the serration pattern has a width of many bubble diameters, then additional means are required to guide the bubbles along the desired specific propagation paths. Such guide rails may consist of a pair of permalloy stripes deposited onto the bubble material, or alternately, they may consist of a pair of raised dams or both sides of the propagation path.
It has been found that a serrate surface topography can be used not only for the propagation of isolated bubbles, but also for the unidirectional propagation of bubble lattices in "bubble lattice devices" discussed in copending U.S. patent application Ser. No. 632,604 filed on Nov. 14, 1975 and assigned to the assignee of the present invention. When applied to bubble lattices relative bubble positions are defined by magnetostatic interactions among the bubbles within the lattice, so that guide rails, or lattic isolation means, are required only along both boundaries of the translatable information storing lattice. Use of a serrate surface topography not only eliminates the need for drive conductors, but also provides for uniformly distributed rather than localized driving forces.
While a serrated surface topography has been disclosed, no method of forming such a structure has been described in the literature. Conventional ion milling techniques employed either perpendicular to the surface or at an angle incident thereto do not in and of themselves yield the desired surface topography.